Photocatalytic coating

ABSTRACT

De-polluting, self-cleaning coating compositions are disclosed which comprise an organic binder having dispersed therein photocatalytic titanium dioxide particles substantially in anatase form which have an average crystallite size between about 1 nm and about 150 nm and which preferably have photocatalytic activity in the presence of visible light. Advantageously, the coatings of the invention do not require pre-activation to achieve high initial photocatalytic activity against pollutants in the air, such as NO x  compounds.

This application is a divisional under 35 U.S.C §120 of U.S. patentapplication Ser. No. 11/848,972, filed Aug. 31, 2007, the contents ofwhich are hereby incorporated by reference in their entirety.

FIELD OF INVENTION

The present invention relates to compositions for imparting aphotocatalytic coating on a surface. More specifically, the inventionrelates to de-polluting, self-cleaning paints comprising titaniumdioxide particles which do not require prior activation to achieve highinitial photocatalytic activity.

BACKGROUND OF THE INVENTION

The photocatalytic properties of the semiconductor material titaniumdioxide result from the promotion of electrons from the valence band tothe conduction band under the influence of ultraviolet (UV) and near-UVradiation. The reactive electron-hole pairs that are created migrate tothe surface of the titanium dioxide particles where the holes oxidizeadsorbed water to produce reactive hydroxyl radicals and the electronsreduce adsorbed oxygen to produce superoxide radicals, both of which candegrade NO_(x) and volatile organic compounds (VOCs) in the air. In viewof these properties, photocatalytic titanium dioxide has been employedin coatings and the like to remove pollutants from the air. Suchcoatings may also have the advantage of being self-cleaning since soil(grease, mildew, mold, algae, etc.) is also oxidized on the surface.

Despite the benefits of existing photocatalytic titanium dioxidecoatings, there is room for improvement in the art. Particularly, it hasbeen observed that the initial activity of conventional photocatalytictitanium dioxide coatings is poor unless the coating has beenpre-activated, such as by washing with water. While not wishing to bebound by any theory, it is believed that the activation step is requiredto remove organic constituents present in the coating composition fromthe surface of the catalyst or possibly to provide a hydrated surface onthe titanium dioxide particles from which reactive radical species areformed. However, this additional step makes application of aphotocatalytic titanium dioxide coating somewhat inconvenient because itis time consuming and adds additional costs to the application process.It would be desirable to provide a photocatalytic titanium dioxidecoating, particularly in the form of a paint, which does not requirepre-activation (e.g., a washing step or exposure to elements) to achievehigh initial activity levels.

It has also been difficult to provide coatings having high levels ofphotocatalyst because the catalyst tends to oxidize and break down thepolymeric binder of the coating. This problem is exacerbated when thecoating is exposed to intense UV radiation from direct sunlight, as isthe case with an exterior paint. Such coatings are often formulated withinorganic binders or with organic polymers which are resistant tophotocatalytic oxidation at relatively low catalyst concentrations.However, in low light conditions the de-pollution properties of thecoating are less than optimal. It would be desirable to provide acoating for use in low light environments (e.g., indoors) thatincorporates high levels of photocatalyst for optimal de-pollution andwhich is resistant to degradation, yet provides high catalytic activityunder indoor lighting conditions.

It is therefore an object of the present invention to provide coatingcompositions, particularly paint compositions, which comprise titaniumdioxide photocatalysts capable of removing pollutants from the air,which photocatalysts have high initial activity without prioractivation. It is a further object of the invention to provide durablecoatings having high levels of photocatalytic titanium dioxide whichcoatings have de-pollution activity in low light environment, and inparticular in the presence of visible light.

The foregoing discussion is presented solely to provide a betterunderstanding of nature of the problems confronting the art and shouldnot be construed in any way as an admission as to prior art nor shouldthe citation of any reference herein be construed as an admission thatsuch reference constitutes “prior art” to the instant application.

SUMMARY OF THE INVENTION

In accordance with the foregoing objectives and others, it hassurprisingly been found that coatings comprising titanium dioxide ofcrystallite size in the range of about 1 nm (nanometers) to about 150nm, more particularly about 5 nm to about 30 nm, and preferably about 5to about 10 nm, do not require pre-activation (e.g., by washing withwater) to achieve a high initial level of photocatalytic activity in thepresence of light. The inventive coatings show substantialphotocatalytic activity in the presence of visible light which makesthem ideal for use as de-polluting coatings in low light environments,including the indoors.

In one aspect of the invention, the self-cleaning, de-polluting coatingcompositions are in the form of water-based paints which include (i)from about 5% to about 40% by volume photocatalytic titanium dioxide,preferably in substantially pure anatase form, the photocatalytictitanium dioxide being characterized by an average crystallite sizebetween about 5 nm and about 30 nm and having photocatalytic activity inthe presence of visible light; (ii) one or more additional pigments,such that the total pigment volume concentration (“PVC”) of the paint,inclusive of said photocatalytic titanium dioxide, is at least about65%; and (iii) a styrene acrylic copolymer binder; the paint beingcapable of substantially reducing NO_(x) compounds in the absence ofprior activation with water.

Another aspect of the invention provides substrates having depositedthereon a layer of the self-cleaning, de-polluting coating compositionsaccording to the invention, and optionally further comprising anovercoat disposed on said paint layer comprising a second photocatalytictitanium dioxide having a crystallite sizes in the range of 5 nm to 30nm, the overcoat being formed by applying a sol over the paint layer.

In another aspect of the invention, a method is provided for removingNO_(x) or other pollutants from the air, comprising applying to asurface, such as a wall, floor, ceiling, or the like, a layer ofde-polluting coating according to the invention, with or without prioractivation by washing with an aqueous solvent, and preferably without awashing step, said coating being capable of substantially removingpollutants from the air in the presence of UV and/or visible light,preferably in the presence of visible light, and optionally applying asol topcoat comprising photocatalytic titanium dioxide over said paintlayer.

These and other aspects of the present invention will be betterunderstood by reference to the following detailed description andaccompanying figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 compares the NO_(x) activities of two photocatalytic titaniumdioxide coatings which have not been pre-activated under variouslighting conditions, where “Comp. 1” is a coating comprisingphotocatalytic titanium dioxide powder having an average crystallitesize of about 5-10 nm and “Comp. 2” is a coating comprisingphotocatalytic titanium dioxide powder having an average crystallitesize of about 15-25 nm.

FIG. 2 compares the NO_(x) activities of various coating systemscomprising a styrene acrylic photocatalytic paint according to theinvention having various photocatalytic titanium dioxide sol topcoats(B-G) disposed thereon.

DETAILED DESCRIPTION

All terms used herein are intended to have their ordinary meaning unlessotherwise provided. All references to “% by weight” herein relate to theweight % of the total paint formulation, including solvent, rather thanthe dried paint, unless otherwise specified. Reference to “% by volume”or “pigment volume concentration” refers to the volume % of the drypaint or coating, unless otherwise specified. The term “NO_(x)” refersto the species NO (nitrogen oxide) and NO₂ (nitrogen dioxide), eithercollectively or individually.

In the broadest sense of the invention, the self-cleaning, de-pollutingcoating compositions comprise photocatalytic titanium dioxide particles,an organic binder, and optionally one or more additional pigments, suchas calcium carbonate. The coatings may be in the form of paints(interior or exterior), in particular water-based paints, and ideallywill have a high (e.g., greater than 60%) total pigment volumeconcentration (“PVC”).

The coatings or paints are capable of substantially reducing NO_(x)compounds in the absence of prior activation with water. It will beunderstood that while the coatings of the invention are capable ofsubstantially reducing pollutants in the absence of prior activationwith water, it is nevertheless within the scope of the invention toactivate the coatings by treatment with water after application tofurther enhance the photocatalytic activity.

Where it is stated that a paint has substantial “initial” photocatalyticactivity, in the absence of prior activation with water, it is meantthat the paint has substantial measurable activity against NO_(x)compounds immediately after a coating of the paint formed onto asubstrate has fully dried and/or cured to the extent customarilypermitted before such a paint is put into service (e.g., it is non-tackyand does not readily transfer on touching, etc.).

Where reference is made to “removal” of pollutants from the air, it willbe understood to include complete or partial removal of pollutants fromthe air. Whether removal is “substantial” can be determined by themethods provided in the examples, where “substantial” removal refers toreduction in the total concentration of a fixed amount of givenpollutant by at least about 2.5%, preferably at least about 5%, and morepreferably at least about 7.5%.

The self-cleaning, depolluting paints of the invention compriseparticles of photocatalytic titanium dioxide (TiO₂) which are capable offorming electron-hole pairs in the presence of electromagneticradiation, particularly ultraviolet (UV), near-UV, and/or visible light.Preferably, the photocatalytic titanium dioxide is capable ofsubstantial photoactivity in the presence of visible light. To this end,it has surprisingly been discovered that careful control over thecrystalline form and particle size of the titanium dioxide providesphotocatalyts which are capable of removing pollutants in low UV lightenvironments, particularly indoor environments, and which havesubstantial initial activity, even in the absence of activation bywashing with a solvent (e.g., water).

The photocatalytic titanium dioxide particles for use in the paintcompositions is preferably predominantly in the anatase crystalline formbecause of its higher photoactivity than the rutile form.“Predominantly” means that the level of anatase in the titanium dioxideparticles of the paint is greater than 50% by mass, although it ispreferred that the level of anatase is greater than about 80%, and morepreferably greater than about 90%. In some embodiments, thephotocatalytic titanium dioxide particles of the paint will be insubstantially pure anatase form, meaning that the content of the rutilecrystalline form is less than about 5%, more particularly, less thanabout 2.5%, and more preferred still, less than about 1% by mass. Insome embodiments, the photocatalytic titanium dioxide particles will befree of the rutile form, meaning that the rutile crystal form is notdetectable by crystallography. Put another way, the photocatalytictitanium dioxide particles may comprise 100% anatase form. The degree ofcrystallization and the nature of the crystalline phase are measured byX-ray diffraction.

The photocatalytic titanium dioxide particles for use in the paintcompositions will typically have an average particle size which enablesthe particles to predominately absorb, rather than scatter, light. Asthe particle sizes become very small, the band gap between the valenceand conduction bands decreases. Thus, with sufficiently small particlesizes, it has been observed that titanium dioxide particles are capableof absorbing light in the visible spectrum. The titanium dioxideparticles for inclusion in the inventive paints will typically have aparticle size between about 1 nm and about 150 nm. More typically, theparticle size will be between about 5 nm and about 20 nm, 25 nm, orabout 30 nm. In a preferred embodiment, the particle size of thetitanium dioxide in the paint will be between about 5 nm and about 15nm, and more particularly between about 5 and about 10 nm. Referenceherein to the size of titanium dioxide particles (or crystallites) willbe understood to mean the average particle size of the titanium dioxideparticulates. Where the particle size is modified by the term “about,”it will be understood to embrace somewhat larger or smaller particlessizes than the indicated value to account for experimental errorsinherent in the measurement and variability between differentmethodologies for measuring particle size, as will be apparent to oneskilled in the art. The diameters may be measured by, for example,transmission electron microscopy (TEM) and also XRD.

Alternatively, the particles may be characterized by surface area.Typically, the powdered titanium dioxide photocatalyst will have asurface area, as measured by any suitable method, including 5-point BET,of greater than about 70 m²/g, more typically, greater than about 100m²/g, and preferably greater than about 150 m²/g. In some embodiments,the titanium dioxide photocatalyst will have a surface area greater thanabout 200 m²/g, greater than about 250 m²/g, or even greater than about300 m²/g.

The photocatalytic titanium dioxides available from Millennium InorganicChemicals under the designations PCS300 and PC500 have been found to beparticularly useful for inclusion in the paints according to theinvention. PCS300 is a 100% anatase titanium dioxide powder having anaverage crystallite size between about 5 nm and about 10 nm. PC500 isalso a 100% anatase titanium dioxide powder, which has a TiO₂ contentbetween about 82% and about 86% by weight, and which has a surface areaof about 250 to about 300 m²/g, as measured by 5-point BET, whichtranslates to an average particle size of about 5 nm to about 10 nm. Theproduct designated PC105, also from Millennium Inorganic Chemicals, willalso find utility in some embodiments of the invention. Thisphotocatalytic powder comprises greater than 95% by weight titaniumdioxde, the TiO₂ being 100% anatase, and has an average crystallite sizeof about 15 nm to about 25 nm and a surface area between about 80 andabout 100 m²/g.

The photocatalytic titanium dioxide will typically comprise from about 2to about 40% by volume of the paint formulation. More typically, thephotocatalytic titanium dioxide will comprise from about 5% to about 20%by volume of the paint, and preferably from about 7.5% to about 15% byvolume. In a representative embodiment, the photocatalytic titaniumdioxide comprises about 10% by volume of the paint formulation. Theforegoing amounts represent the volume of photocatalyst in the finalpaint formulation (e.g., including solvent), rather than the volumepercentage in the dried paint coating. Typically, the weight percent oftitanium dioxide in the paint formulation will be between about 1% byweight and about 20% by weight, more typically between about 5 and about10% by weight, and preferably about 7.5% by weight.

It is within the scope of the invention to provide paints having two ormore different titanium dioxide photocatalysts, where at least one, andpreferably each, of the titanium dioxide photocatalyst materials meetthe specifications described above. Thus, for example, the inventionembraces the use of bimodal photocatalytic titanium dioxide material,formed by combining two different titanium dioxide powders or sols,wherein at least one, and preferably both, have a particle size and/orsurface area as defined above. In other embodiments, the photocatalystwill “consist essentially of” a particular titanium dioxide materialdescribed herein, by which is meant any additional photocatalyst havingmaterially different activities is excluded, or that amounts ofadditional photocatalyst which materially impact the durability,de-polluting, or self-cleaning properties of the paint are excluded.

The paints of the invention comprise an organic binder. In the broadestaspect of the invention, it is contemplated that any polymeric bindermay be employed. In one embodiment, the polymeric binder is awater-dispersible polymer, including but not limited to latex binders,such as natural latex, neoprene latex, nitrile latex, acrylic latex,vinyl acrylic latex, styrene acrylic latex, styrene butadiene latex, andthe like. Exemplary polymers for these compositions include, but are notlimited to, methyl methacrylate, styrene, methacrylic acid2-hydroxyethyl acrylate polymer (CAS #70677-00-8), acrylic acid, methylmethacrylate, styrene, hydroxyethyl acrylate, butyl acrylate polymer(CAS #7732-38-6), butyl acrylate, methyl methacrylate, hydroxyethylacrylate polymer (CAS #25951-38-6), butyl acrylate, 2-ethylhexylacrylate, methyl methacrylate, acrylic acid polymer (CAS #42398-14-1),styrene, butylacrylate polymer (CAS #25767-47-9), butyl acrylate,2-ethylhexyl acrylate, methacrylic acid polymer C (CAS #31071-53-1),acrylic polymers, and carboxylated styrene butadiene polymers to name afew. Combinations of more than one organic binder are also contemplatedto be useful in the practice of the invention.

In particular, the organic binder may be chosen among copolymers ofstyrene/butadiene, and polymers and copolymers of esters of acrylic acidand in particular copolymers of polyvinylacrylic and styrene/acrylicesters. In the present invention, styrene acrylic copolymer includescopolymers of styrene/acrylic esters thereof. The styrene acrylicemulsion sold under the tradename ACRONAL™ 290D (BASF) has been found tobe particularly useful as an organic binder in the inventive paints.

In some embodiments, the organic binder in the inventive paints will“consist essentially of” the preferred styrene acrylic binder, by whichis meant that the presence of additional organic binders in amountswhich materially reduce the durability of the paint coating on asubstrate, as compared to an otherwise identical paint coatingcomprising only styrene acrylic binder as the organic binder, areexcluded.

In some embodiments, the inventive paints will be substantially free ofinorganic binders, by which is meant that the levels of inorganic binderis not sufficient to form a continuous adherent film on a substrate, inthe absence of organic binder. In representative embodiments, the paintscomprise less than 0.5% by weight, preferably less than about 0.2% byweight, and more preferred still, less than about 0.1% by weightinorganic binders. In some embodiments, the inventive paints are free ofinorganic binders. Inorganic binders include, without limitation, alkalimetal silicates such as, for example, potassium silicate, sodiumsilicate, and/or lithium silicate.

The paints according to the invention may further comprise one or morepigments. The term “pigments” is intended to embrace, withoutlimitation, pigmentary compounds employed as colorants, including whitepigments, as well as ingredients commonly known in the art as“opacifying agent” and “fillers.” Included are any particulate organicor inorganic compound able to provide hiding power to the coating, andin particular at least one inorganic compound like non-photocatalytictitanium dioxide. Such titanium dioxide pigments which are notphotoactive are disclosed in U.S. Pat. No. 6,342,099 (MillenniumInorganic Chemicals Inc.), the disclosure of which is herebyincorporated by reference. In particular, the titanium dioxide pigmentmay be the particles of Tiona™ 595 sold by Millennium InorganicChemicals Ltd. Pigments also include calcium carbonate, which istypically added to paint as a filler. One suitable calcium carbonatematerial is that sold under the tradename Setacarb™ 850 OC (Omya).

The paints according to the invention typically, but not necessarily,have a pigment volume concentration (PVC) between about 60% and about90%, more typically between about 65% and about 80%, and preferablybetween about 70% and about 75%. The term “pigment volume concentration”refers to the total percentage by volume of all pigments in thecomposition, wherein the term “pigment” includes all forms of titaniumdioxide, whether photocatalytic (e.g., PC500) or non-photocatalytic(e.g., Tiona™ 595), as well as any other components generally regardedin the art as pigments, including without limitation calcium carbonateand other particulate fillers.

If necessary, various other compounds may be added to the composition ofthe invention, but preferably such an addition does not compromise theshelf life, photoactivity, durability or non-staining properties of theresulting coating. Examples of such additional compounds includefiller(s) such as quartz, calcite, clay, talc, barite and/orNa—Al-silicate, and the like; pigments like TiO₂, lithopone, and otherinorganic pigments; dispersants such as polyphosphates, polyacrylates,phosphonates, naphthene and lignin sulfonates, to name a few; wettingagents, including anionic, cationic, amphoteric and/or non-ionicsurfactants; defoamers such as, for example, silicon emulsions,hydrocarbons, and long-chain alcohols; stabilizers, including forexample, mostly cationic compounds; coalescing agents including, withoutlimitation, alkali-stable esters, glycols, and hydrocarbons; rheologicaladditives like cellulose derivatives (e.g., carboxymethylcelluloseand/or hydroxyethylcellulose), xanthane gum, polyurethane, polyacrylate,modified starch, bentone and other lamellar silicates; water repellentssuch as alkyl siliconates, siloxanes, wax emulsions, fatty acid Lisalts; and conventional fungicide or biocide.

Example 1

The ability of the inventive coatings to remove NO_(x) pollutants, itsself-cleaning properties, and its durability was investigated bypreparing three water-based styrene acrylic paints. Comparative samples“Comp. 1” and “Comp. 2” each comprised 10% photocatalytic titaniumdioxide by volume, whereas no photocatalyst was present in the controlsample. The photocatalytic titanium dioxide used in Comp. 1 was PCS300from Millennium Inorganic Chemicals. PCS300 is a photocatalytic titaniumdioxide powder having an average crystallite size of about 5 to about 10nm (nanometers). The photocatalytic titanium dioxide used in Comp. 2 wasPC105, also from Millennium Inorganic Chemicals, which has an averagecrystallite size of about 15-25 nm. PCS300 and PC105 both have ananatase content of about 100%. The complete paint formulations areprovided in Table 1.

TABLE 1 Comp. 1 Comp. 2 Control Ingredient Function Weight (g) Part AWater solvent 159.94 159.94 152.41 Natrosol 250MR thickener 99.30 99.30104.64 Foammaster NXA antifoaming agent 0.60 0.60 0.63 Antiprex Adispersant 3.30 3.30 3.48 Tiona T595 TiO₂ pigment 70.58 70.58 74.37PC105 TiO₂ photocatalyst — 47.06 — PCS300 TiO₂ photocatalyst 47.06 — —Setacarb 850 OG filler (CaCO₃) 145.28 145.28 186.55 Part B Acronal 290Dstyrene acrylic 69.86 69.86 73.62 Texanol coalescent 3.46 3.46 3.67Acticide SPX bacteriocide 0.60 0.60 0.63 Total (weight) 600.00 600.00600.00

The remaining components of Table 1 are as follows: The thickener is a3% solution of hydroxyethylcellulose sold under the designationNatrosol™ 250 MR (Hercules). The antifoaming agent Foammaster™ NXA isproprietary, sold by Henkel Corp. Setacarb™ 850 OG is a calciumcarbonate filler obtained from Omya. Antiprex™ A is water-solublepolymer dispersant from Ciba Specialty Chemicals. Tiona™ T595 ispigmentary titanium dioxide from Millennium Inorganic Chemicals.Acronal™ 290D is a styrene acrylic copolymer latex used as an organicbinder available from BASF. Acronal™ 290D comprises 50% by weight solidsin water. Texanol™ is an ester alcohol coalescing solvent sold byEastman Kodak. Acticide SPX is a bacteriocide from Acti Chem SpecialtiesInc.

The Part A and Part B ingredients were separately mixed under high shearmixing. Part A was then added to Part B under high shear mixing to formthe finished paints. Each paint sample is applied at a coverage of 770g/m² (based on the dried weight of the coating) on a substrate and thesubstrates were submitted to the following tests.

I—Determination of NO_(x) Removal by Coatings

The complete methodology for determining NOx removal is described inU.S. Patent Pub. 2007/0167551, the disclosure of which is herebyincorporated by reference. Briefly, the samples were placed in anair-tight sample chamber and sealed. The sample chamber is incommunication with a three channel gas mixer (Brooks Instruments,Holland) through which NO (nitric oxide), NO₂ (nitrogen dioxide), andcompressed air containing water vapor are introduced into the chamber atpredetermined levels. The samples are irradiated with 8 W/m² UVradiation in the range of 300 to 400 nm from a UV Lamp Model VL-6LM 365& 312 nanometer wavelengths (BDH). Initial values and final values(after five minutes irradiation) of NOx were measured by a NitrogenOxides Analyser Model ML9841B (Monitor Europe) connected to the samplechamber. The % reduction in NOx was measured as (ΔNOx/Initial NOx)×100.Each sample was investigation without pre-activation and withpre-activation (after washing with water). The results are summarized inTable 2.

TABLE 2 no pre-activation pre-activated Sample % NO_(x) Reduction Sample% NO_(x) Reduction Comp. 1 58.6 Comp. 1 68.3 Comp. 2 8.3 Comp. 1 55.2Control 0 Control 0

The results indicate that the paint comprising photocatalytic titaniumdioxide powder having an average crystallite size of about 5 to about 10nm (Comp. 1) exhibits a surprisingly high NOx activity even without theconventional washing step to pre-activate the photocatalyts. Bycomparison, Comp. 2 which comprises titanium dioxide powder having anaverage crystallite size of about 15 nm to about 25 nm exhibits a farlesser degree of NOx reduction in the absence of a pre-activation step.Comp. 1 and Comp. 2 both display excellent NOx removal properties afterwashing to pre-activate the catalyst. However, Comp. 1 with nopre-activation was unexpectedly superior to Comp. 2 even in the casewhere the Comp. 2 sample was pre-activated.

II—Determination of Coating Photoactivity Towards Methylene Blue

The methodology employed for determining photoactivity toward methyleneblue is similar to that described in U.S. Patent Pub. 2007/0167551, thedisclosure of which is hereby incorporated by reference, and is modifiedas described herein. The self-cleaning properties of each paint samplewere investigated based on their ability to degrade the organic dyemethylene blue. As the dye is degrades to water, carbon dioxide, andnitrogen containing species, a loss of color is observed. Thephotoactivity is monitored by measuring L* (brightness). The protocol isas follows:

Prepare a film of the paint on a suitable substrate such as Melinexfilm, aluminium panel, or glass plate. The film thickness should besimilar to that used in the final application and generally not lessthan 25 microns thick when dry. The paint film is allowed to dry atleast overnight.

Prepare a solution of methylene blue in water by dissolving 0.3739 g inone liter of water to give a concentration of 1 mmol/L. Pour themethylene blue solution into a suitable dish in which to immerse thepaint film. Soak the paint films in the methylene blue solution for 30to 60 minutes to ensure that the methylene blue is chemically absorbedonto the surface of the TiO₂.

Remove the paint film from the solution and remove excess with absorbenttissue. Thoroughly dry the paint films and then measure the brightness(L*) value using a colorimeter or spectrophotometer.

Expose the paint films to UV light for a period of between 18 to 48hours at an intensity of 30 to 60 W/m² (300-400 nm wavelengths) such asin an Atlas Suntest cabinet.

Re-measure the L* value. The difference between the initial and final L*measurements is a measure of the self-cleaning power of the coating. Thelarger the difference in L* value the greater the self-cleaning effect.The results for each paint after 18 hours and 36 hours of irradiationare shown below in Table 3.

TABLE 3 ΔL* Sample 18 hours 36 hours Comp. 1 15.3 18.2 Comp. 2 10.6 12.5Control 0 0

The results indicate that the paint comprising photocatalytic titaniumdioxide powder having an average crystallite size of about 5 to about 10nm (Comp. 1) exhibits substantially greater self-cleaning activity thanthe Comp. 2 sample after 18 hour and 36 hours of irradiation.

III—Determination of Coating Durability

The complete methodology for determining durability of the paints isdescribed in U.S. Patent Pub. 2007/0167551, the disclosure of which ishereby incorporated by reference. The methodology involves acceleratedweathering of 20 to 50 micron thick paint films on a stainless steelsubstrate in a Ci65A Weatherometer (Atlas Electric Devices, Chicago)under a 6.5 kW Xenon source emitting 550 W/m² UV at 340 nm. The sampleswere heated to about 63° C. and water spray was applied for 18 minutesout of every 120 minutes, with no dark cycle. The durability is measuredas a function of the weight loss of the sample following exposure.

Table 4 summarizes the results for durability testing for Comp. 1 andComp. 2 at various time intervals up to 1,551 hours.

TABLE 4 Comp. 1 Comp. 2 hours Weight loss (%) 0 0.0 0.0 286 24.6 21.1451 38.7 33.5 586 48.6 43.3 765 59.6 55.5 997 70.0 69.6 1,181 76.7 80.11,365 83.4 84.6 1,551 88.9 90.7

As shown in Table 4, the durability of the Comp. 2 paint issubstantially identical to the durability of the less photoactive Comp.1 paint after about 1,000 hours of exposure. This result was unexpectedas it would have been anticipated that the more highly photoactive paintof Comp. 2 would have deteriorated substantially more rapidly than theless active Comp 1. under these conditions. It is noted that through 765hours the % weight loss was marginally greater for the more active Comp.1 paint with the maximum difference observed after about 451 hours. Thisis likely due to the fact that Comp. 1 has a much greater initialactivity without pre-activation as compared to Comp. 2 (see Table 2).However, during weathering, both paints become fully activated, due tothe presence of water, and the % weight loss is seen to converge atlonger intervals. Over the entire period of accelerated weathering,Comp. 1 exhibited excellent durability which was comparable to Comp. 2.

III—Determination of NOx Removal Under Different Light Sources

The procedure for determination of NO_(x) removal, described above inpart I of this Example, was employed to determine the respectiveabilities of paint samples Comp. 1 and Comp. 2 to remove NOx underdifferent light sources. In addition to UV, low intensity fluorescentstrip lighting, day light (as filtered through glass), and Osramincandescent light sources were employed. In each case, the paints weretested without prior activation. The results are tabulated below (Table5) and illustrated in FIG. 1.

TABLE 5 Comp. 1 Comp. 2 Light Source % NO_(x) Reduction UV 61.6 14.1Fluorescent strip 9.1 0.0 Daylight 22.4 1.0 Incandescent 7.8 0.0

The UV light was from a UV Lamp Model VL-6LM 365 & 312 nanometerwavelengths (BDH) as employed in part I of this Example. The fluorescentlight was light produced from conventional indoor fluorescent striplighting. The daylight was filtered through glass to provide anintensity of 2.4 microW/cm². The incandescent light was provided by anOsram incandescent lamp.

The results shown in Table 5 demonstrate that the Comp. 1 paint displayssubstantial NOx removal activity, without pre-activation, under each ofthe lighting sources, whereas the Comp. 2 paint, in the absence ofpre-activation, has no activity under fluorescent strip or incandescentlighting and insubstantial activity in daylight (2.4 microW/cm²). Theexcellent performance of the Comp. 1 paint under these ultra-low UVlighting conditions is believed to arise due to the ability of thePCS300 photocatalyst to absorb in the visible spectrum. Without wishingto be bound by any particular theory, it is believed that the very smallcrystallite size (e.g., about 5-10 nm) results in a decrease in the bandgap between the valence and conduction bands, thereby allowing theparticles to create electron-hole pairs in the presence of visiblelight.

Example 2

While paints having photocatalyst crystallite sizes between about 5 andabout 15 nm represent a preferred embodiment of the invention,including, for example, the paint designated Comp. 1 in Example 1 havinga photocatalytic TiO₂ particle size of about 5-10 nm, the benefits ofhigh PVC (pigment volume concentration) achievable through the use of astyrene acrylic binder are also seen, albeit more modestly, with lesspreferred titanium dioxide crystallite sizes (i.e., about 15 to about 50nm). For example, paints employing high levels of PC105 photocatalyst(about 15 nm to about 25 nm crystallite size) will also find utility incoatings for removing NO_(x).

This example illustrates the efficacy of the paint designated Comp. 2 inExample 1 in removing pollutants under “real world” conditions. A cornerof a parking garage was sealed off by constructing two walls to providea 917 m³ closed area with a ceiling height of 2.85 m. The 322 m² ceilingsurface was coated with the Comp. 2 paint of Example 1 while the walls(existing and artificial) were covered with nylon. The photocatalyticpaint was not pre-activated by washing with water. During the NO_(x)removal experiments, the enclosure was illuminated by twenty UV lampsfixed symmetrically 20 cm from the ceiling to provide a total UVirradiance of 1 W/m².

The exhaust from a vehicle placed outside of the enclosure was connectedby a pipe to the enclosed area such that exhaust gases were released4.74 m inside the enclosure. Ventilation (inlet and outlet) was providedin the room through the artificial walls in order to maximize theconcentration of pollutants near the ceiling and to provide an airflowand velocity of 566 m³/h and 14.3 m/h, respectively. The airflow andvelocity of exhaust gas from the car were estimated to be 50.6 m³/h and2 m/s, respectively, such that a positive pressure was maintained in theenclosed space in order to avoid the inflow of air from outside theenclosure.

The NO_(x) exhaust gases from the car were continuously measured using aportable gas analyzer. NOx measurements were also taken continuously atthe inlet and outlet ventilator and at a third sampling point near theceiling about 15 in from the outlet ventilator.

After the exhaust gas was allowed to reach a steady state in theenclosure (approximately 3 hours), the UV lamps were turned on for fouror five hours. The reduction in NO and NO₂ was measured as thedifference between the steady state concentration and the finalconcentration after irradiation. The values were corrected for thedecrease in NO concentration and the increase in NO₂ concentration inthe car exhaust over the test period in order to isolate thecontribution of the photocatalytic paint to the total reduction in thesepollutants. The experiments were repeated over three consecutive days.On the fourth day, control measurements were taken in the absence of UVirradiation. The results are shown in Table 6 (% NO photocatalyticdegradation) and Table 7 (% NO2 photocatalytic degradation).

TABLE 6 Initial NO % NO concentration UV Final NO reduction % NOExperimental at steady state Irradiation concentration Total % NO in cardegradation Day (ppb) time (h) (ppb) removed emission due to TiO₂ 1 10925 581 46.8 28 18.8 2 623 5 351 43.6 28 15.6 3 1286 4 898 30.2 23.5 6.7 41151 0 829   28 (5 h)   28 (5 h) 0 880 23.5 (4 h) 23.5 (4 h)

TABLE 7 Initial NO₂ % NO concentration UV Final NO₂ Total % increase %NO Experimental at steady state Irradiation concentration NO₂ in cardegradation Day (ppb) time (h) (ppb) removed emission due to TiO₂ 1 8925 767 14 8.5 22.5 2 879 5 708 19.4 8.5 27.9 3 1110 4 1059 4.6 8.5 13.1 41031 0 1119 8.5 8.5 0

It is evident from the data in Tables 6 and 7 that a styrene acrylicpaint comprising about 15-25 nm average size photocatalytic titaniumdioxide crystallites at a level of 10% by volume (about 8% by weight) iseffective in reducing NO_(x) pollutants from the air, even in theabsence of prior activation. Further, this example highlights theusefulness of the inventive paint coating in applications such a parkinggarage interiors where it is desirable to remove concentrated pollutantsfrom the air.

Example 3

A styrene acrylic paint was prepared substantially as described inExample 1 except that PCS300 was replaced with a comparable 100% anatasephotocatalytic titanium dioxide powder available from MillenniumInorganic Chemicals under the trade designation PC500. PC500 has asurface area of about 300 m²/g which translates to an averagecrystallite size of about 5 to about 10 nm. PC500 was included in thepaint at a level of 8% by volume and the styrene acrylic bindercomprised about 50% by volume. The ability of this paint to remove NOxwithout prior activation was studied as a function of UV intensityacross a range of intensities from 0.5 W/m² to 8 W/m² according to theprocedure described above in Example 1. The results are given in Table8.

TABLE 8 UV intensity (W/m²) % NO_(x) reduction 0.5 31.3 1 37.1 2 40.6 344.2 4 45.5 5 46.4 6 46.9 7 46.9 8 47.3

These results demonstrate that even at very low UV intensities, theinventive paints provide high removal of pollutants, even withoutpre-activation. In fact, the difference in NO_(x) reduction was only 16%(47.3%−31.3%) over more than one order of magnitude increase in UVintensity.

The PC500 paint was over-coated with various photocatalytic TiO₂ solslisted in Table 9 to investigate whether further improvements in thede-NO_(x) properties could be attained.

TABLE 9 Sample Sol topcoat A none B S5300A C SP300N D S5300B (23.6% w/wTiO₂) E S5300B (10.0% w/w TiO₂) F S5300B (5.0% w/w TiO₂) G AW1610 (0.24%w/w TiO₂)

Sample A represents styrene acrylic paint comprising PC500 photocatalystwithout any sol topcoat. Samples B-G represent the paint of sample Ahaving the indicated sol topcoat applied thereto. S5300A is aphotocatalytic titanium dioxide sol available from Millennium InorganicChemicals. It is an aqueous colloidal dispersions of ultrafine TiO₂(anatase) peptised with acid at a pH of about 1.1 (±0.4), having atitanium dioxide content of about 20 (±2) % by weight, a density ofabout 1.2 g/ml, and a surface area greater than 250 m²/g by 5-point BET(on dried product). S5300B, also available from Millennium InorganicChemicals, is also an aqueous colloidal dispersions of ultrafine TiO₂(anatase) peptised with base at a pH of about 11.4 (±1), having atitanium dioxide content of about 17.5 (±2.5) % by weight, a density ofabout 1.1 g/ml, and a surface area greater than 250 m²/g by 5-point BET(on dried product). The various S5300B sols in Table 9 were modified tohave the indicated titanium dioxide contents on a weight basis. AW1610is a sol comprising photocatalytic TiO₂ having an average crystallitesize of about 3.6 nm, pH of 9.2, a density of about 1.00 g/ml, and aTiO₂ content of about 0.25%. SP300N is a slurry of photocatalytic TiO₂(about 17% by weight) having an average crystallite size of about 5-10nm, pH of 7.0, and a density of about 1.15 g/ml.

The ability of each coating system (paint+sol) to remove NO_(x) wasinvestigated as a function of UV light intensity from 0.5 W/m² to 8W/m². The results are shown in FIG. 2. As can be seen, coating system Dcomprising the PC500 paint with an overcoat of S5300B (23.6% w/w TiO₂)showed unexpectedly superior de-NO_(x) across the entire range of UVintensities with only minimal variation in % NO_(x) reduction across therange.

All references including patent applications and publications citedherein are incorporated herein by reference in their entirety and forall purposes to the same extent as if each individual publication orpatent or patent application was specifically and individually indicatedto be incorporated by reference in its entirety for all purposes. Manymodifications and variations of this invention can be made withoutdeparting from its spirit and scope, as will be apparent to thoseskilled in the art. The specific embodiments described herein areoffered by way of example only, and the invention is to be limited onlyby the terms of the appended claims, along with the full scope ofequivalents to which such claims are entitled.

The invention claimed is:
 1. A method of removing NO_(x) compounds fromthe air comprising applying to a surface a layer of de-polluting coatingcomprising photocatalytic titanium dioxide, one or more additionalpigments, and an organic latex binder comprising a styrene acryliccopolymer, said photocatalytic titanium dioxide being characterized byan average crystallite size between about 5 nm and about 10 nm, asurface area greater than 250 m²/g, and having photocatalytic activityin the presence of visible light, said coating being capable ofsubstantially removing NO_(x) compounds from the air in the presence ofvisible light, without a step of first washing the coating with water toactivate it.
 2. The method of claim 1 wherein said coating issubstantially free of inorganic binder.
 3. The method of claim 1 whereinsaid coating is free of inorganic binder.